A Good Month for Asteroids

Asteroid hunters have enjoyed a close-up look at
two new potentially hazardous space rocks as they passed close
to the Earth in September.

September
20, 2000 -- This has been a good month for astronomers studying
Near-Earth asteroids (NEAs). No fewer than five sizable
space rocks have flown past our planet since the beginning of
September -- three of them in the last four days.

There was no danger of a collision at any time, say researchers.
All of the asteroids missed our planet by comfortable margins
of 11 to 75 lunar distances. Still, by cosmic standards, they
were close at hand.

Among the parade were two asteroids, 2000 QW7 and 2000
RD53, that were brand-new discoveries. NASA's Near Earth
Asteroid Tracking system spotted 2000 QW7 on August 26,
2000 -- less than a week before its
closest approach. Then, 11 days later on Sept. 6th, MIT's
LINEAR program detected 2000 RD53. Both were passing by
our planet no farther away than 12 times the distance to the
Moon. As news of the discoveries spread, astronomers rushed to
their telescopes for a closer look.

"Dozens of observers, including many skilled amateurs,
are monitoring these bright objects," says Eleanor Helin,
the principal investigator for JPL's Near-Earth Asteroid Tracking
(NEAT) program. "The
close approaches of these asteroids offer a rare opportunity
to learn about their physical characteristics, including what
they're made of and their rotational periods," she added.

Above: NASA's 70-meter diameter Goldstone radar is the
largest and most sensitive Deep Space Network antenna. Astronomers
are using the Goldstone antenna to bounce radio signals off Near-Earth
asteroids this month. [more
information]

2000 QW7 and 2000 RD53 belong to a group known
as "Potentially Hazardous Asteroids" or PHAs. There
are 271 known
PHAs, which by definition are asteroids larger than about
150 meters that can come closer to Earth than 0.05 AU (about
20 lunar distances). In spite of their menacing name, none of
the PHAs we know of now are destined for a collision with our
planet. But that could change.

"PHA orbits can be chaotic. Perturbations -- such
as a gravitational nudge from Mars or Earth -- could change their
orbits. We have to monitor them -- it's highly recommended,"
remarked Helin with a note of understatement.

Â September 2000 Near-Earth Asteroids

Asteroid

DATE
mmm-DD HH:MM

R
(AU)

Vr
(km/s)

H
(Vm)

D
(km)

2000 QW7

Sep-01

12:54

0.0317

6.48

19.5

0.3-0.7

2000 ET70

Sep-04

10:39

0.1895

12.84

18.2

0.6-1.4

2000 RD53

Sep-17

13:20

0.0288

7.77

20.0

0.3-0.6

2000 DP107

Sep-19

13:20

0.0478

12.35

17.9

0.7-1.5

2000 QS7

Sep-20

04:54

0.0872

10.28

20.7

0.2-0.5

Legend: R is the asteroid's
miss distance in AU (astronomical units) on the indicated DATE.
For comparison, the distance between the Earth and the Moon is
approximately 0.0026 AU. Vr is the relative velocity between
the Earth and the asteroid at the time of the flyby. H
is the asteroid's absolute magnitude (the visual magnitude an
observer on Earth would record if the asteroid were placed 1
AU away). D is the size of the asteroid estimated
from its absolute magnitude.

A group of astronomers led by Jean-Luc Margot of the National
Astronomy and Ionosphere Center detected radar echoes from 2000
QW7 and 2000 RD53 as they passed over the powerful
Arecibo radar in Puerto Rico and NASA's Goldstone radar in the
Mojave desert. By analyzing
the echoes, researchers can construct three-dimensional maps
of the asteroids and reduce the uncertainty of their orbital
elements.

"Goldstone radar observations of 2000 QW7 and
RD53 permitted velocity measurements accurate to better
than 4 millimeters per second," says Jon Giorgini, a
senior engineer in JPL's Solar System Dynamics Group "For
2000 RD53, it was possible to make direct range measurements
to the asteroid, from both Arecibo and Goldstone planetary radars,
accurate to at least 300-400 feet."

With a precise orbit determined by radar data, Giorgini
ran 2000 RD53's motion backwards and found that it had
made an even closer approach at 9.3 lunar
distances in 1933, but no one saw it. The next close encounter
as near as this week's won't come until 2198.

According to Giorgini's calculations, 2000 QW7 will
be back sooner than 2000 RD53. On Sept. 15, 2019, it will
pass our planet 14 times farther away than the Moon -- about
the same distance as this week's encounter.

"Evidence from the radar data suggests that 2000
QW7 is a slow rotator," added Jean-Luc Margot. "Its
spin period is on the order of days, which is a puzzle for an
object this size."

Collisions within the asteroid belt are expected to give space
rocks plenty of spin, but 2000 QW7 joins at least two
other NEAs (1999 JM8 and Toutatis) that rotate
slowly.

"Various exotic possibilities have been proposed to
explain how NEAs could lose their angular momentum,"
continued Margot. "These include close encounters with
a planet, tidal despinning of a binary system, or disruption
from a larger asteroid. Obtaining these radar measurements will
help us understand ... their dynamical history."

Above: Astronomers at the Arecibo Observatory detected
strong radar echoes from 2000 RD53 within 90 seconds of their
first look at the asteroid. The frequency offset in this echo
spectrum provides a highly accurate measurement of the line-of-sight
velocity of the object. Image courtesy Jean-Luc Margot.

Slow rotation is just one of many puzzles attending Potentially
Hazardous Asteroids, says Brian Marsden, director of the Minor
Planet Center. Researchers still don't know how many PHAs inhabit
the inner solar system, what they're made of, or exactly where
they come from.

"There are two obvious possibilities," says
Marsden. "PHAs could come from the asteroid belt or they
might be inert comets. Undoubtedly it's a mixture of the two,
but we don't know the fractions."

"We would expect some to be bona fide rocky
asteroids," he continued. "After all, there
are mechanisms that can bring main belt asteroids into Earth-crossing
orbits. The principal one, involving what are called secular
planetary perturbations, takes millions to hundreds of millions
of years -- a short time compared to the age of the Solar System."

"As for the comets, it may be that
they can masquerade as asteroids after their ices have been vaporized
by solar heating. Is there enough particulate material for such
a spent comet to remain coherent, or does it break up? Comet
LINEAR broke up very nicely (when it passed by the Sun earlier
this year)! An alternative is that a rocky crust might completely
cover the comet's ice. We just don't know."

Left: This image captured by the Hubble Space Telescope
shows Comet LINEAR disintegrating after it passed close to the
Sun earlier this year. Could some comets hold together after
all their ices vaporize? If so, they could be masquerading as
Near-Earth asteroids.

Distinguishing between cometary and rocky NEAs is important
in case we ever need to nudge one away from our planet. One of
the most-often discussed scenarios for diverting a PHA involves
launching a nuclear-armed rocket to intercept it. Exploding the
warhead in the wrong spot could have unintended consequences.
Scientists caution that a hailstorm of asteroidal fragments could
be worse than one big piece -- like being hit by a shotgun instead
of a rifle. Knowing what PHAs are made of and how they are put
together is vital.

It's also vital to find such asteroids as early as possible.
2000 QW7 and 2000 RD53 didn't provide much advance
warning. They were discovered 5 and 11 days, respectively, before
their closest approaches to Earth.

"We can miss bright asteroids like 2000 QW7
for several reasons," explains Helin. "For instance,
if an asteroid moves across the Milky Way during its closest
approach, it might be hard to identify among the densely-packed
background stars. Or if the asteroid is close to the Sun as it
approaches, it could be lost in the Sun's glare. Many NEAs (like
QW7 and RD53) are in highly elliptical orbits and
spend most of their time as dim specks beyond the orbit of Mars.
They brighten only as they come close to the Sun, then we can
see them from Earth."

Right: This 1.5
MB MPEG video of 2000 QW7 was captured by John Rogers using
a 0.30 meter Schmidt-Cassegrain telescope at the Camarillo Observatory,
55 miles north of Los Angeles. Amateur
videos of RD53 are also available from spaceweather.com.

"The way to look at these objects is over a long period
of time," noted Marsden. "Yes, some will come
close to Earth and miss us -- as 2000 QW7 and RD53
have done -- but it's the subsequent passes that we have
to worry about. Follow-up observations to establish precise orbits
are very important if we wish to predict future encounters."

"Radar helps a very great deal (in refining asteroid
orbits). We can also look for newly discovered asteroids in old
images, and that helps, too. Nowadays we look for pre-discovery
images as a matter of course."

Indeed, soon after NEAT identified 2000 QW7, a colleague
of Marsden's found this asteroid in early-August data from MIT's
LINEAR search program. Such prediscovery observations, and in
particular confirmed "precovery" observations from
an earlier year, can immediately refine an asteroid's orbit and
make continued tracking easier.

"LINEAR did record 2000 QW7 almost a month
before its closest approach," says Marsden, "but
poor weather limited the observations to a few images on a single
night, and it was moving too slowly to be picked out as unusual."
It's another example of how search programs can miss PHAs. When
they are faint, far away, and moving slowly against the background
stars, PHAs can appear to be harmless main belt objects.

"We're accumulating asteroids at a furious rate,"
says Marsden. "At the turn of the century we knew of
only 500 minor planets; now we've cataloged 17,349 with excellent
orbit determinations. The rate of discovery is approximately
doubling every two years."

The rate could increase further if a
new British government initiative to identify hazardous asteroids
bears fruit. With more telescopes on the lookout, astronomers
will undoubtedly enjoy many more -- and perhaps uncomfortably
numerous -- opportunities for close-up studies of these ever-scary
space rocks.

The Anatomy of Asteroid
Names

Sometimes it seems that astronomers
enjoy picking inscrutable names for the objects they study. What
person on the street would guess that "2000 QW7" is
a fascinating space rock? Nevertheless, there is a method to
this naming madness.

So many new asteroids are discovered
each month that astronomers need an efficient way to catalog
them. The first part of "2000 QW7" is simple -- it
identifies the year of the asteroid's discovery (2000).

Then comes "QW7." The
first letter tells us that the object was identified during the
second half of August. Each half-month is identified with a letter
of the alphabet. January 1st-15th = "A"; January 16th-31st
= "B"; August 16th-31st = "Q", etc. The letter
"I" is omitted in this system.

The second and third characters
"W7" are a shorthand way of counting the number of
asteroids found during the 2nd half of August 2000. The first
asteroid discovered was "2000 QA"; the second was "2000
QB;" The second letter cycles through the alphabet until
it reaches "Z" and then it goes back to the beginning
with an extra number. So, the 26th asteroid discovered during
the second half of August 2000 was "2000 QA1". Remember
that "I" is omitted, so "A1" corresponds
to the 26th asteroid, not the 27th. This means that 2000 QW7
was the 197th asteroid found in the second half of August 2000!

NEAT is managed by JPL for
NASA's Office of Space Science, Washington, DC. The National
Astronomy and Ionosphere Center is operated by Cornell University
under a cooperative agreement with the National Science Foundation
and with additional support from NASA.